1. Field of the Invention
This invention relates to a process for stably and continuously producing a solid-liquid metal mixture in which non-dendritic primary solid particles are dispersed into the remaining liquid matrix (hereinafter referred to as a semi-solidified metal composition) and an apparatus used therefor.
The term "semi-solidified metal composition" used herein means that molten metal (generally molten alloy) is vigorously agitated while cooling to (1) convert dendrites produced in the remaining liquid matrix into a spheroidal or granular shape such that dendritic branches are substantially eliminated or reduced creating non-dendritic primary solid particles) and (2) disperse these primary solid particles into the liquid matrix.
2. Related Art Statement
The semi-solidified metal composition develops excellent working properties at subsequent steps such as casting or the like as well as excellent quality of cast articles as the non-dendritic primary solid particles dispersed in the liquid matrix become increasingly fine. In the production of the semi-solidified metal composition, therefore, it is required to satisfy the following two conditions:
(1) vigorous agitation capable of breaking and separating dendrites to fine non-dendritic primary solid particles in which dendritic branches eliminated or reduced to create particles of a generally spheroidal or granular shape;
(2) strong cooling capable of maximizing the cooling rate.
However, the viscosity increase together with the increase of fraction solid in the production of the semi-solidified metal composition renders it difficult to continuously discharge the semi-solidified metal composition from the production apparatus and, finally, the discharge becomes impossible.
As a process for continuously producing such a semi-solidified metal composition, Japanese Patent Application Publication No. 56-20944 discloses a process wherein molten metal is vigorously agitated in a cylindrical cooling agitation vessel through high rotation of an agitator while cooling to convert dendrites produced in the remaining liquid matrix into non-dendritic primary solid particles in which dendritic branches are eliminated or reduced to create particles of a spheroidal or granular shape, and then these non-dendritic primary solid particles are dispersed into the liquid matrix to form a slurry of semi-solidified metal composition, which is continuously discharged from a nozzle arranged at the bottom of the cooling agitation vessel.
In this process, molten metal is charged into a clearance between the high-speed rotating agitator having a vertically rotational axis and the coaxially arranged cylindrical cooling agitation vessel, molten metal is changed into a semi-solidified state through proper cooling and vigorous agitation in the vessel, and then the molten melt is continuously discharged from the nozzle as a semi-solidified metal composition. According to this process, the cooling rate is undesirably restricted to not more than 2.degree. C./s (in the case of Al-10% Cu alloy) to prevent clogging in the clearance due to the formation and growth of solidification shell on the cooled wall face. And also, it is difficult to control the agitating degree, cooling rate and discharging rate due to the growth of the solidification shell.
The inventors have examined the above technique and confirmed the following problems:
(i) In order to enhance the agitating effect, it is effective to increase the revolution number of the rotating agitator or make the clearance between the cooling agitation vessel and the agitator arranged therein small. However, when the revolution number is increased, the liquid matrix strongly tends to separate away from the agitator through centrifugal force and increases the risk of entrapping gas. And also, the increase of the revolution number is critical in view of the structural strength.
On the other hand, when the clearance is made small, the solidification shell is easily formed and the viscosity resistance increases, so that the clearance can not be made small in practical use.
(ii) When a strong cooling means is adopted for increasing the cooling rate, the solidification shell formed on the cooled wall face causes adhesion to the agitator, whereby the operation is rendered impossible.
(iii) In non-steady heat transfer such as initial operation stage or the like, it is difficult to control temperature, and hence the adhesion of solidification shell to the agitator may be caused due to excessive cooling. That is, it is difficult to stably start the operation.
(iv) When the semi-solidified metal composition is discharged under gravity, the force for passing the semi-solidified metal composition through the clearance between the cooling agitation vessel and the agitator is only a pressure based on the gravity, so that the discharging is rendered impossible when the fraction solid in the semi-solidified metal composition increases to raise the viscosity.